In the exploration for and production of oil and gas reservoirs, various types of logging tools are lowered into the well for the purpose of determining the physical characteristics of rock surrounding the well. Often, it is important to know with a great deal of accuracy the depth of the logging tool. It may also be important to accurately know the distance between locations which have been occupied by the tool.
One such tool is a borehole gravity meter. A borehole gravity meter is used to estimate the interval bulk density of rock surrounding the well. When the borehole gravity meter is raised or lowered in the well, the change in the relative value of the vertical component of the intensity of the gravitational field can be accurately determined. If the distance between the vertical locations is also accurately known, the interval bulk density can be accurately determined.
The present technology for determining tool location utilizes a mechanical length measurement device at the surface. Odometers, radioactive tape, inking, painting, and mechanical wheels have been described as being useful in determining the length of wire which has entered the hole (e.g., see "Cable Marker/Detector is a Magnetic-Type System", Oil and Gas Journal, Feb. 21, 1982, page 148). These devices do not account for downhole cable stretch due to friction, tool and cable weight, or thermal expansion or shrinkage. These variations in cable length can introduce an error of several feet in absolute tool location or several inches in the relative distance between two locations in a borehole.
The use of laser light in determining the pressure and temperature conditions within a wellbore has been described (U.S. Pat. No. 4,375,164 incorporated herein by reference) wherein the index of refraction of two optical fibers are compared by examining the interference pattern of laser light in a photomultiplier.
In none of the above technology is an accurate means of determining the vertical location of a tool or logging sonde within a wellbore described in which the interference pattern of a laser light is utilized.
Consequently, it is an object of this invention to provide a method and means for accurately determining location of a tool in a borehole or casing. It is another object of the invention to provide a method and means for accurately determining the change in location of a tool in a borehole or casing. It is a further object of the invention to provide a method and means of accurately determining the interval bulk density of rocks which occupy the space between two locations. It is a further object to accurately measure the depth at which data samples are taken such as resistivity, sonic travel time, etc.